Wire bonds having pressure-absorbing balls

ABSTRACT

A semiconductor device with a chip having at least one metallic bond pad ( 101 ) over weak insulating material ( 102 ). In contact with this bond pad is a flattened metal ball ( 104 ) made of at least 99.999% pure metal such as gold, copper, or silver. The diameter ( 104   a ) of the flattened ball is less than or equal to the diameter ( 103   a ) of the bond pad. A wire ( 110 ) is connected to the bond pad so that the wire has a thickened portion ( 111 ) conductively attached to the flattened metal ball. The wire is preferably made of composed metal such as gold alloy. The composition of the flattened ball is softer than the wire. This softness of the flattened ball protects the underlying insulator against damage caused by pressure or stress, when the composed ball is attached.

FIELD OF THE INVENTION

The present invention is related in general to the field ofsemiconductor devices and processes and more specifically to structureand method of metal bonds to contact pads over dielectrics.

DESCRIPTION OF THE RELATED ART

It is well known in semiconductor technology that bond pads on siliconintegrated circuits can be damaged during conventional thermosonic wirebonding to circuit bond pad made of aluminum or copper. Mechanicalloading and ultrasonic stresses applied by the tip of the bondingcapillary to the bond pad are particularly suspect. When the damage isnot apparent during the bonding process, the defects may manifestthemselves subsequently by succumbing to thermo-mechanical stressesgenerated during plastic encapsulation, accelerated reliability testing,temperature cycling, and device operation. The damage appears in mostcases as microcracks which may progress to fatal fractures in theunderlying dielectric, as chip-outs of brittle or mechanically weakdielectric films, often together with pieces of metal or silicon, or aslifted ball bonds or delamination of metal layers.

Recent technological developments in the semiconductor industry tend toaggravate the problem. For instance, newer dielectric materials such assilicon-containing hydrogen silsesquioxane (HSQ) are being preferred dueto their lower dielectric constant which helps to reduce the capacitanceC in the RC time constant and thus allows higher circuit speed. Sincethe density and porosity of dielectric films affect the dielectricconstant through absorption or desorption of water, films with thesecharacteristics are introduced even when they are mechanically weaker.Films made of aerogels, organic polyimides, and parylenes fall into thesame category. These materials are less dense and mechanically weakerthan previous standard insulators such as the plasma-enhanced chemicalvapor deposited dielectrics. This trend even affects stacks ofdielectric layers such as alternating layers of plasma-generatedtetraethylorthosilicate (TEOS) oxide and HSQ, or ozone TEOS oxide andHSQ. Since these materials are also used under the bond pad metal, theymagnify the risk of device failure by cracking.

In addition, the pitch of bond pads is being progressively more reducedto save valuable silicon real estate. Consequently, the bondingparameters have to become more aggressive to achieve stronger bonds inspite of smaller size. Bonding force and ultrasonic energy duringbonding are being increased. Again, the risk of yield loss and loweredreliability is enhanced.

Attempts to solve these problems by using softer wires and attachmentsballs have failed because these wires sag under their own weight inlong-looping spans, or can be swept in the transfer molding process. Inaddition, these difficulties tend to get aggravated, when larger wirediameters are needed to keep the electrical resistance of long loops atan acceptable value. Furthermore, bonding balls made from theselarge-diameter wires would end up with ball diameters too large for theever shrinking pad sizes.

SUMMARY OF THE INVENTION

A need has therefore arisen for a low cost, reliable wire bonding methodcombining simultaneously small ball sizes with reliable intermetallics,low risk of damaging the dielectrics under the bond pad during thebonding process, avoiding sagging of long wire spans, and keeping theelectrical resistance of these long wire spans at acceptable values.There is a technical advantage, when the method is flexible enough to beapplied for different semiconductor product families and a wide spectrumof design and assembly parameters, and also achieves improvements towardthe goals of improved process yield and device reliability. These arefurther technical advantages when these innovations are accomplishedusing the installed equipment base so that no investment in newmanufacturing machines is needed.

One embodiment of the invention is a semiconductor device with a chip,which has at least one metallic bond pad over insulating material. Incontact with this bond pad is a flattened metal ball made of at least99.999% pure metal such as gold, copper, or silver. The diameter of theflattened ball is less than or equal to the diameter of the bond pad. Awire is connected to the bond pad so that the wire has a thickenedportion conductively attached to the flattened metal ball. The wire ispreferably made of composed metal such as gold alloy, copper alloy, orcopper-coated gold; the composition of the flattened ball is softer thanthe wire. This softness of the flattened ball protects the underlyinginsulator against damage caused by pressure or stress, when the composedball is attached, even when the insulator includes mechanically weakdielectrics.

The device may also have an electrically conducting line spaced from thebond pad by a gap. The wire is spanning this gap by means of a loop ofcontrolled shape, whereby the bond pad is connected to the conductingline. Due to the hardness of the composed wire, the loop does not sageven for long spans. Furthermore, the wire has a diameter selected sothat the electrical resistance of the loop can be kept below apre-determined value.

Another embodiment of the invention is a method for fabricating asemiconductor device, which has a chip with at least one bond pad overinsulator material. A flattened metal ball is formed in contact with thebond pad. A wire is then connected to the bond pad by forming athickened portion of the wire and conductively attaching the thickenedportion to the flattened metal ball, which has a composition softer thanthe wire.

Another embodiment of the invention is a method for fabricating asemiconductor device, which starts by providing a semiconductor chipwith at least one metallic bond pad over insulator material. A firstfree air ball is formed at the tip of a first capillary, which is loadedwith a wire of at least 99.999% pure metal. The freshly formed firstfree air ball is then attached to the bond pad, flattened by means ofthe first capillary, and separated from the pure wire. A second free airball is formed at the tip of a second capillary, which is loaded with awire of composed metal. The freshly firmed second free air ball is thenattached to the flattened first ball and flattened by means of thesecond capillary; the composed wire is finally spanned into a controlledloop. The diameter of the composed wire is selected so that the loop hasa predetermined electrical resistance.

It is a technical advantage of the invention to eliminate restrictionson the size of the dielectric pattern, thus minimizing the risks ofinflicting cracking damage even to very brittle dielectrics.

It is another technical advantage of the invention to provide designconcepts and process methods which are flexible so that they can beapplied to several families of products, and are general, so that theycan be applied to several generations of products. The inventionsupports the ongoing miniaturization trend in the semiconductorindustry, especially the shrinking of bond pad diameters.

Another object of the invention is to use only processes alreadyemployed in the fabrication of integrated circuit devices, thus avoidingthe cost of new capital investment and using the installed fabricationequipment base.

The technical advances represented by certain embodiments of theinvention will become apparent from the following description of thepreferred embodiments of the invention, when considered in conjunctionwith the accompanying drawings and the novel features set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of a contact pad on a semiconductordevice with metals attached according to an embodiment of the invention.

FIG. 2 is a schematic cross section of a contact pad on a semiconductordevice after the first step of a contact fabrication method according toan embodiment of the invention.

FIG. 3 is a schematic cross section of a completed bonding wire loophaving a contact to a semiconductor bond pad fabricated according to theinvention.

FIG. 4 is a schematic cross section of two adjacent contact pads afterthe first step of a contact fabrication method according to anembodiment of the invention.

FIG. 5 is a schematic cross section of two adjacent contact pads aftercompleting the contact fabrication according to an embodiment of theinvention.

FIG. 6 is a schematic cross section of two adjacent contact padsillustrating the improved process tolerance gained by the contactfabrication method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The schematic cross section of FIG. 1 illustrates a portion on thesurface of a semiconductor device, which has a protective overcoat 103,typically consisting of silicon nitride or oxynitride. A window in thisovercoat exposes a metallic bond pad 101. The window has a diameter 103a. Bond pad 101 is located over insulating material 102. The bond padmetal 101 may be aluminum or copper, or stacked layers thereof. Theinsulator 102 may include mechanically weak dielectrics such assilicon-containing hydrogen silsesquioxane (HSQ), aerogels, organicpolyimides, parlenes, or alternating layers of plasma-generatedtetraethylorthosilicate (TEOS) oxide and HQS, or ozone TEOS oxide andHSQ. The mechanical weakness of these dielectrics, generally referred toas low-k dielectrics, is prone to suffer cracks by mechanical pressureand stress created during the wire bonding operations, but their lowerdielectric constant helps to reduce the capacitance C in the RC timeconstant of circuits and thus allows higher circuit speed.

A flattened metal ball 104 is in contact with bond pad 101. In thisembodiment, the diameter 104 a of ball 104 is less than or equal todiameter 103 a of the overcoat window and the flattened metal ball ismade of metal at least 99.999% pure, such as pure gold, copper, orsilver. These metals are commercially available by many companies, forinstance, Nittetu Micro Metal, Japan, under the brand name 5N, orDegussa, Germany. At these high purity levels, these metals are verysoft, especially when they are macro-crystalline (heat treated).

FIG. 1 further shows a wire 110 connected to the bond pad, wherein thewire has a diameter 110 a and a thickened portion 111. Preferably,thickened portion 111 is created from a free air ball at the end of wire110, by flattening this air ball. The thickened portion 111 isconductively attached to the flattened metal ball 104. Wire 110 is acomposed metal such as gold alloy, copper alloy, or copper-coated gold,and has typically a diameter of about 20 to 25 μm. These wires arecommercially available by many companies, for instance, Nittetu MicroMetal, Japan, under the brand name NT. In this composition, metal 110 isharder than the soft metal 104. These wires will, therefore, not sageven at long wire loops, and by choosing a large enough diameter, theywill also have an electrical resistance lower than a predeterminedvalue.

The cross section of FIG. 2 illustrates schematically a preferred methodof fabricating a flattened ball 201 from a wire 202 of diameter 202 a byemploying the generally practiced wire ball bonding technology. Diameter202 a of wire 202 is preferably smaller than diameter 110 a of bondingwire 110.

The preferred wire bonding process begins by positioning thesemiconductor chip (only portion 200 shown in FIG. 2) on a heatedpedestal to raise the temperature to between 150 and 300° C. The goldwire 202 is strung through a capillary of a diameter suitable for wireddiameter 202 a and a predetermined orifice. At the tip of the wire, afree air ball is created using either a flame or a spark technique. Theball has a typical diameter from about 1.2 to 1.6 wire diameters. Thecapillary is moved towards the chip bonding pad 210 and the ball ispressed against the metallization of the pad. Generally, a combinationof compression force and ultrasonic energy create the formation ofintermetallics between ball metal and pad metal and thus a strongmetallurgical bond. The compression (also called Z- or mash) force istypically between about 17 and 75 g; the ultrasonic time between about10 and 20 ms; the ultrasonic power between about 20 and 50 mW. At timeof bonding, the temperature usually ranges from 150 to 270° C. For goldwire on copper pad, metal interdiffusion takes place in order togenerate the strong weld. The exact outline of the flattened ball onFIG. 2 is a function of the shape of the capillary orifice; the ballwill be more flattened by the impact of the thickened wire end (ball)(111 in FIG. 1).

With the soft flattened ball 201 protecting the bond pad and itsunderlying dielectrics, the air ball 111 (FIG. 1) formed from wire 110can be metallurgically attached with the required ultrasonic energy andpressure. In this action, ball 201 is flattened approximately into theshape shown for ball 104 in FIG. 1. After flattened ball 111 isattached, automated wire bonding allows tightly controlled shape of thewire loop.

As schematically illustrated in the example of FIG. 3, an electricallyconducting line 301, or leadframe segment, is spaced from bond pad 101by a gap 302. The gap 302 is bridged by wire loop 303. Moving thecapillary in a predetermined and computer-controlled manner through theair will create a wire looping of exactly defined shape. For instance,with a modern wire bonder, rounded, trapezoidal, linear and customizedloop paths can be formed without sagging. The electrical resistance ofthe whole loop 303 is kept under a predetermined value by the suitablylarge selected diameter 110 a of wire 110. Finally, the capillaryreaches its desired destination at conductive line or leadframe segment301. The capillary is lowered to touch the line at 304; with the imprintof the capillary, a metallurgical stitch bond is formed, and the wire isbroken off to release the capillary. Stitch contacts are small yetreliable; the lateral dimension of the stitch imprint is about 1.5 to 3times the wire diameter (its exact shape depends on the shape of thecapillary used, such as capillary wall thickness and capillaryfootprint).

FIG. 4 shows two bond pads 401 and 402 in close proximity; the ongoingtrend in semiconductor technology is that the pad pitch 403center-to-center is shrinking. It is a technical advantage of thepresent invention that it supports this trend towards fine pad pitch,because it provides metal stud contacts in the form of soft flattenedballs 404 and 405, which may have a diameter smaller than or equal tothe diameter of pads 401 and 402. The flattened balls 404 and 405 havebeen made from soft metal wire of small enough diameter 406.

FIGS. 5 and 6 illustrate how the present invention supports flattenedcontact balls with a diameter as large as, or even slightly larger than,the bond pads. In FIG. 5, two bond pads are designated 501 and 502,respectively, having a pitch center-to-center 503. The flattened, softcontact balls of small diameter, made from highly pure wire of smalldiameter, are designated 504 and 505, respectively. The wire 510 ofharder metal and larger diameter 510 a provides flattened balls 511 and512 having a diameter as large as, or even slightly larger than, thediameter of pads 501 and 502. The large diameter of balls 511 and 512can be tolerated because of the height 520 of the underlying flattenedsift balls 504 and 505. FIG. 5 illustrates a flattened ball 511 and 512in good alignment with contact pads 501 and 502, indicated by the dashedcenter lines.

In FIG. 6, however, the automated bonder of the capillary with the hardwire was misaligned relative to the pad locations, and thus producedflattened contact balls 611 and 612 having center lines systematicallymisaligned relative to the center lines of the pads. The misalignment isdesignated 630. In spite of this mishappening, no electrical shortbetween neighboring pads 601 and 602 is produced due to the height 620of the flattened soft balls 604 and 605 under the balls 611 and 612. Theinvention thus supports process forgiveness in the device fabrication.

Another embodiment of the invention is the preferred method forfabricating a semiconductor device, which comprises the following steps:Providing a semiconductor chip having at least one metallic bond padover insulator material; the insulator may include mechanically weaklow-k dielectrics. Next, forming a first free air ball at the tip of afirst capillary loaded with an at least 99.999% pure metal wire;examples include gold, copper, and silver wire. Next, attaching thefreshly formed first free air ball to the bond pad, flattening the firstball by means of the first capillary, and separating the flattened firstball from the pure wire.

The step of attaching the first free air ball may include mechanicalpressure and ultrasonic energy without damaging mechanically weak low-kdielectrics under the bond pad, because of the softness of the puremetal. The step of separating the flattened ball may consist of aflame-off process. It is preferred that the flattened first ball has adiameter smaller than or equal to the diameter of the bond pad.

In the next process step, a second free air ball is formed at the tip ofa second capillary loaded with a composed metal wire. Examples ofcomposed metals are alloys, such as gold alloy, coated wires, such ascopper-coated gold, and process-hardened wires. The freshly formedsecond free air ball is then attached to the flattened first ball; inthis process, the second ball is flattened by means of said secondcapillary. The diameter of the second flattened ball may beapproximately the diameter of the bond pad. The step of attaching thesecond free air ball may include mechanical pressure and ultrasonicenergy, but they are less likely to damage mechanically weak low-kdielectrics under the bond pad, because the flattened first ball madeout of soft metal acts as a pressure-absorbing cushion.

Subsequently, the composed wire is spanned into a controlled loop, whichbridges a gap between the bond pad an electrically conductive line or aleadframe segment. The diameter of the composed wire is selected so thatthe loop has an electrical resistance equal to or below a certainpredetermined value. This means, the diameter of the composed-metal wiremay be larger than the diameter of the pure-metal wire.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription.

As an example, the invention covers integrated circuits made in silicon,silicon germanium, gallium arsenide, or any other semiconductor materialused in integrated circuit manufacture.

As another example, in some devices, the dielectrics are fortified bymetal-grid structures. For these strengthened insulators, a firstflattened ball made of lightly less pure metal, for example, 99.99% or99.9%, may be satisfactory.

It is therefore intended that the appended claims encompass any suchmodifications or embodiments.

1. A method for fabricating a semiconductor device, comprising the stepsof: providing a semiconductor chip having at least one metallic bond padover insulator material; forming a first free air ball of a first metalwire having a first composition at the tip of a first capillary;attaching said first free air ball to said bond pad, and separating saidfirst ball from said first metal wire; providing a second metal wirehaving a second composition harder than the first composition; forming asecond free air ball of the second metal wire at the tip of a capillary;and attaching said second free air ball to said first ball.
 2. Themethod according to claim 1 further comprising the step of spanning saidcomposed wire into a controlled loop to an electrically conducting linespaced from said bond pad by a gap and attaching the end of said loop tosaid line so that said loop bridges said gap.
 3. The method according toclaim 1 wherein said process steps of attaching said air balls includemechanical pressure and ultrasonic energy supplied by said capillaries.4. The method according to claim 1 wherein said second wire is selectedfrom a group of wires consisting of alloys, coated wires, andprocess-hardened wires.
 5. The method according to claim 1 wherein saidbond pad has an area and said first free air ball has a bottom areasmaller than or equal to said bond pad area.
 6. The method according toclaim 1 wherein said step of separating said first ball from said purewire is performed by flaming off said wire.
 7. The method according toclaim 2 wherein the diameter of said second wire is selected so thatsaid controlled loop has a predetermined electrical resistance.
 8. Themethod according to claim 7 wherein said diameter of said second wire islarger than the diameter of said first wire.
 9. A method for fabricatinga semiconductor device, comprising the steps of: providing asemiconductor chip having at least one metallic bond pad over aninsulator material; forming a metal ball in contact with said bond padfrom a first metal wire with a first composition; and connecting asecond wire having a second composition harder than the firstcomposition to said bond pad by forming a thickened portion of said wireon said metal ball.
 10. The method of claim 9, in which the ball is agold ball of at least 99% purity.
 11. The method of claim 9, in whichthe ball is a gold ball of at least 99.999% purity.
 12. The method ofclaim 1, in which the first wire is a gold wire of at least 99% purity.13. The method of claim 1, in which the first wire is a gold wire of atleast 99.999% purity.